Conventionally, there have been known apparatus for estimating a cylinder-interior air quantity (quantity of air having been introduced into a cylinder of an internal combustion engine) by making use of a physical model modeling the behavior of air flowing through the intake passage of the internal combustion engine.
Japanese Patent Application Laid-Open (kokai) No. 2003-184613 discloses one of such apparatus. The disclosed apparatus uses the physical model in which the cylinder-interior air quantity to be estimated is represented by equations including terms regarding a pressure and a temperature of air upstream of a throttle valve (throttle-valve upstream air) and regarding the pressure and temperature of air downstream of the throttle valve (throttle-valve downstream air). Accordingly, the cylinder-interior air quantity cannot be accurately estimated unless the pressure and temperature of throttle-valve upstream air are accurately estimated.
Incidentally, in a naturally aspirated internal combustion engine to which the above-described conventional apparatus is applied, the pressure and temperature of throttle-valve upstream air are generally equal to those of atmospheric air. Accordingly, in the conventional apparatus, a pressure and a temperature detected by an intake-air pressure sensor and an intake-air temperature sensor disposed in an intake passage upstream of a throttle valve are employed as the pressure and temperature of throttle-valve upstream air.
Meanwhile, in some cases, a turbocharger is provided on an internal combustion engine in order to increase the maximum output of the engine. The turbocharger includes a compressor disposed upstream of a throttle valve within an intake passage. In such an internal combustion engine, since air downstream of the compressor (throttle-valve upstream air) is compressed upon operation of the compressor, the pressure and temperature of the throttle-valve upstream air suddenly vary as compared with those of atmospheric air. Therefore, possibly, the cylinder-interior air quantity cannot be accurately estimated when a pressure and a temperature detected by an intake-air pressure sensor and an intake-air temperature sensor are employed as the pressure and temperature of the throttle-valve upstream air.
A conceivable solution is to construct a physical model on the basis of the conservation law regarding air within the intake passage extending from the compressor to the throttle valve (throttle-valve upstream section) and to estimate the pressure and temperature of throttle-valve upstream air by means of the constructed physical model. In general, in accordance with a physical model constructed on the basis of the conservation law regarding air within a certain space, the pressure and temperature of air within the space can be represented by an equation including terms regarding the flow rate of air flowing into the space. Accordingly, in order to accurately estimate the pressure and temperature of throttle-valve upstream air by use of the above-described physical model, the flow rate of air flowing out of the compressor (compressor-outflow-air flow rate) must be obtained accurately.
Incidentally, this compressor-outflow-air flow rate can be considered to be equal to a compressor-inflow-air flow rate which is the flow rate of air flowing into the compressor. Accordingly, the compressor-outflow-air flow rate may be obtained by detecting the compressor-inflow-air flow rate by use of a hot-wire air flowmeter, which has been conventionally disposed in the intake passage upstream of the compressor, and employing the detected compressor-inflow-air flow rate as the compressor-outflow-air flow rate.
However, the flow rate of air detected by the hot-wire air flowmeter involves a time delay in relation to the actual flow rate of air, the time delay stemming from time required for transfer of heat between air and the hot wire and time required to heat the hot wire. Such detection delay occurs not only when a hot-wire air flowmeter is used but also when the other type of air flowmeter is used. Accordingly, when the compressor-inflow-air flow rate varies within a short period of time; for example, a transition period during which the operation conditions (load, engine speed, etc.) vary, there arises a problem that the pressure and temperature of throttle-valve upstream air cannot be accurately estimated even when the detected compressor-inflow-air flow rate is employed as the compressor-outflow-air flow rate, because the compressor-inflow-air flow rate detected by means of the air flowmeter greatly differs from the actual compressor-inflow-air flow rate.
Accordingly, an object of the present invention is to provide an air quantity estimation apparatus for an internal combustion engine equipped with a turbocharger, which apparatus can accurately estimate the compressor-inflow-air flow rate by use of an air flowmeter inverse model which compensates for the detection delay of an air flowmeter, to thereby accurately estimate the cylinder-interior air quantity.